This invention relates to the inspection of surface and subsurface flaws and discontinuities in objects by ultrasonic nondestructive testing, and is particularly concerned with the provision of a novel improved ultrasonic couplant composition having a minimum of components to provide good ultrasonic transmission and particularly adapted for use for such ultrasonic inspection, such composition being biodegradable, stable and heat resistant; and also to a method of ultrasonic inspection employing such novel composition.
Ultrasonic nondestructive testing of bodies such as metal parts has been developed. This is a method which utilizes UHF (ultra-high frequency) sound waves to detect discontinuities in parts. An ultrasonic testing device is employed, in which a probe or transducer is placed in contact with a surface of the part to be inspected. Ultrasonic waves are generated by applying a pulsed oscillating voltage from a pulser to a transducer (piezoelectric crystal). When the transducer is electrically excited and is adequately coupled to a part being inspected, an ultrasonic wave passes into the part. A change in acoustic properties of the part (surface and subsurface cracks, discontinuities, part surfaces, interfaces) reflects the wave back to the transducer. The reflected wave mechanically stresses the transducer and the transducer generates electrical charges. The electrical signals are applied to an amplifier circuit with the ultrasonic instrument, where they are amplified and displayed on a CRT (cathode-ray tube).
In the above procedure, a couplant is required to be applied to a surface of the object to be tested, to provide an effective medium for ultrasonic transmission between the transducer or probe applied to the surface of the body, and the body undergoing nondestructive testing. Thus, the primary purpose of couplants is to provide a suitable ultrasonic path between the transducer and part being inspected. Air is a poor conductor of ultrasonic energy. The couplant also fills in and smooths out irregularities of a part's surface and aids in movement of the transducer. A further purpose of the couplant is to serve as an acoustic impedance matching medium. The closer the couplant acoustic impedance matches that of the part being inspected, the better the ultrasonic wave transfer.
Ultrasonic coupling compositions which have been employed to date by the industry include for example, water, glycerin, light oil and petroleum jelly or grease. These materials have been used in the past basically because of their relatively good ultrasonic transmission characteristics. However, none of the prior art couplants such as those noted above have all of the desirable features required for efficient ultrasonic inspection of parts. Thus, for example, water and glycerin are corrosive to certain metals. Oils and greases usually contaminate the surface of the parts to be tested, making it necessary to degrease the parts after testing. Further, in those cases where the parts comprise titanium or its alloys, and degreasing is required, the use of highly flammable solvents which are hazardous, is necessary for this purpose, since the usual chlorinated solvents employed for degreasing are detrimental to titanium and cannot be employed. Also, certain commercially available gelled couplants are unstable and tend to dry up, particularly at elevated temperatures, leaving powdery residues. The odor level of certain prior art couplants also is undesirable.
An additional criterion has recently developed also with respect to ultrasonic couplant compositions. Generally, couplants presently being used and containing solvents and wetting agents present a disposal problem in that they are substantially non-biodegradable, that is, they are very difficult to decompose by bacteria in sewage disposal plants. Hence the necessity for the development of ultrasonic couplant compositions which are biodegradable, that is which employ vehicles which are biodegradable, and are readily available despite the petrochemical shortage, has attained considerable importance.
In my above U.S. Pat. Nos. 3,915,885 and 3,915,886 there is disclosed novel dye penetrant compositions which are biodegradable, containing as the vehicle for the dye, certain biodegradable nonionic oxyalkylated alcohols.
In my U.S. Pat. No. 3,826,127 there is disclosed an improved ultrasonic couplant composition in the form of an aqueous gel containing N-methyl-2-pyrrolidone, a water soluble surfactant, e.g. a nonyl phenyl ether of polyethylene glycol, and silica. Although such couplant composition has been found effective and is stable up to certain elevated temperatures, e.g. of the order of about 125.degree. F., at substantially higher temperatures it loses volatiles and tends to dry out. Also, the patent composition is not biodegradable.
It is accordingly an object of the present invention to provide an ultrasonic couplant composition, that is a composition for use in ultrasonic inspection of parts, having excellent ultrasonic transmission, which is noncorrosive to metals, particularly titanium, aluminum and steel, and their various alloys, is nonflammable, odorless and essentially nontoxic, can be simply applied to a part to be tested and the part surface oriented in a horizontal, vertical or overhead position, without dripping of the composition, and while permitting a probe or transducer to remain adhered to the couplant composition regardless of the orientation of the part surface, such couplant composition being essentially nonhygroscopic, easily removable from the part surface such as by removal with water, and being reusable and relatively inexpensive to manufacture.
Another important object of the present invention is the provision of a biodegradable heat resistant ultrasonic couplant composition which is a simple formulation and which does not require the use of mixtures of conventional solvents and wetting agents, and which is formed of an essentially single or sole vehicle in the form of a biodegradable nonionic surfactant, and which is highly heat stable and does not become powdery at relatively high temperatures. A still further object is the provision of procedure employing such novel heat stable biodegradable ultrasonic couplant composition for inspection of cracks, flaws and metallurgical conditions in structural components, particularly parts heated to elevated temperature.